Pharmacogenomics — MCQs

Pharmacogenomics Indian Medical PG Practice Questions and MCQs

Question 31: Therapeutic drug monitoring of plasma concentrations of antihypertensive drugs is not practiced because:

A. Determination of plasma level of these drugs is a tedious and long process.
B. It is easier to measure the therapeutic effect of these drugs. (Correct Answer)
C. The antihypertensive effect does not increase linearly with the dose.
D. All antihypertensive drugs are prodrugs.

Explanation: ### Explanation **1. Why Option B is Correct:** The primary goal of Therapeutic Drug Monitoring (TDM) is to guide dosage when the clinical effect is difficult to measure or when there is a narrow therapeutic index. For antihypertensive drugs, the therapeutic effect—**blood pressure (BP)**—is an easily measurable, objective, and non-invasive clinical endpoint. Since a physician can directly titrate the dose based on a sphygmomanometer reading to achieve the target BP, measuring plasma concentrations is unnecessary, expensive, and redundant. **2. Analysis of Incorrect Options:** * **Option A:** While some assays are complex, modern chromatography (HPLC/LC-MS) makes measuring drug levels routine. The lack of TDM is due to clinical irrelevance, not technical difficulty. * **Option B:** This is the standard pharmacological principle: if a "surrogate marker" or "clinical effect" is easily quantifiable (e.g., BP for hypertension, INR for warfarin, Blood Glucose for insulin), TDM is not required. * **Option C:** While some drugs show non-linear kinetics, this would actually be an *indication* for TDM, not a reason to avoid it. * **Option D:** Only a few antihypertensives are prodrugs (e.g., Enalapril, Ramipril, Methyldopa). Most (e.g., Amlodipine, Atenolol) are active drugs. **3. NEET-PG High-Yield Pearls:** * **Indications for TDM:** Narrow therapeutic index (Lithium, Digoxin, Theophylline), lack of easily measurable clinical effect (Antiepileptics, Antipsychotics), or suspected toxicity/non-compliance. * **Exceptions to TDM:** Do not perform TDM if the clinical effect is easily measured (BP for antihypertensives, Blood sugar for Antidiabetics, INR for Warfarin). * **Key Concept:** TDM is most useful when there is a strong correlation between plasma concentration and clinical effect, but a poor correlation between dose and plasma concentration.

Question 32: Which antitubercular drug metabolism is under genetic control?

A. Rifampicin
B. Isoniazid (Correct Answer)
C. Cyclosporine
D. Pyrazinamide

Explanation: **Explanation:** The correct answer is **Isoniazid (INH)**. Its metabolism is a classic example of pharmacogenetics, specifically involving the **N-acetyltransferase 2 (NAT2)** enzyme in the liver. **1. Why Isoniazid is Correct:** Isoniazid is metabolized via **acetylation**. Due to genetic polymorphism in the *NAT2* gene, individuals are categorized into two phenotypes: * **Fast Acetylators:** They metabolize the drug quickly, leading to lower plasma levels and a higher risk of **treatment failure** or sub-therapeutic responses. * **Slow Acetylators:** They metabolize the drug slowly, leading to higher plasma levels and an increased risk of **peripheral neuropathy** (due to Vitamin B6 deficiency) and drug-induced lupus. **2. Why other options are incorrect:** * **Rifampicin:** It is primarily metabolized by deacetylation and is a potent inducer of CYP450 enzymes, but its metabolism is not significantly dictated by a specific genetic polymorphism like INH. * **Cyclosporine:** This is an immunosuppressant (not an antitubercular drug) metabolized by CYP3A4. While CYP3A4 has variations, it is not the classic example of genetic control in the context of TB therapy. * **Pyrazinamide:** It is converted to pyrazinoic acid by the enzyme pyrazinamidase. While resistance can occur due to bacterial gene mutations (*pncA*), its human metabolism is not a major focus of clinical pharmacogenetics. **High-Yield Clinical Pearls for NEET-PG:** * **Acetylation** also affects the metabolism of **Hydralazine, Procainamide, and Sulfonamides** (Mnemonic: **SHIP** - Sulfonamides, Hydralazine, Isoniazid, Procainamide). * **Slow acetylators** are more prone to **Drug-Induced Lupus Erythematosus (DILE)**. * To prevent peripheral neuropathy in slow acetylators taking INH, always co-administer **Pyridoxine (Vitamin B6)** at 10–50 mg/day.

Question 33: Pharmacogenetics is associated with:

A. Variability of enzyme action
B. Individual variability in oral absorption
C. Different dose-response relationships in different individuals
D. All of these (Correct Answer)

Explanation: **Explanation:** **Pharmacogenetics** is the study of how genetic variations (polymorphisms) influence an individual’s response to drugs [1]. It encompasses both **pharmacokinetics** (what the body does to the drug) and **pharmacodynamics** (what the drug does to the body). 1. **Variability of enzyme action (Option A):** This is the most classic example of pharmacogenetics [2]. Genetic polymorphisms in Phase I (e.g., CYP2D6, CYP2C19) and Phase II (e.g., NAT2, TPMT) enzymes lead to "fast" or "slow" metabolizer phenotypes, significantly altering drug levels [1]. 2. **Individual variability in oral absorption (Option B):** Genetic variations in drug transporters, such as **P-glycoprotein (MDR1)** or Organic Anion Transporting Polypeptides (OATPs) in the gut epithelium, dictate how much of a drug enters the systemic circulation [1]. 3. **Different dose-response relationships (Option C):** This refers to pharmacodynamic variability. Mutations in drug targets (receptors, ion channels, or enzymes) can change a drug’s affinity or efficacy. For example, polymorphisms in the **VKORC1** gene alter the sensitivity to Warfarin, requiring different doses to achieve the same anticoagulant effect. Since genetic factors influence absorption, metabolism, and receptor sensitivity, **Option D** is the correct answer. **High-Yield Clinical Pearls for NEET-PG:** * **Slow Acetylators (NAT2 deficiency):** Prone to peripheral neuropathy with **Isoniazid** and drug-induced Lupus with **Hydralazine/Procainamide** [1]. * **Pseudocholinesterase deficiency:** Leads to prolonged apnea after **Succinylcholine** administration. * **TPMT deficiency:** Increases the risk of life-threatening bone marrow suppression with **6-Mercaptopurine** and **Azathioprine**. * **HLA-B*1502:** Strongly associated with Stevens-Johnson Syndrome (SJS) in patients taking **Carbamazepine**.

Question 34: Therapeutic index of a drug is a measure of its?

A. Safety (Correct Answer)
B. Efficacy
C. Potency
D. Selectivity

Explanation: ### Explanation **1. Why "Safety" is Correct:** The **Therapeutic Index (TI)** is a quantitative measurement of the relative safety of a drug. It represents the ratio between the dose that causes toxicity and the dose that produces the desired therapeutic effect. Mathematically, it is expressed as: **TI = TD₅₀ / ED₅₀** (or LD₅₀ / ED₅₀ in animal studies) * **TD₅₀:** Dose that produces a toxic effect in 50% of the population. * **ED₅₀:** Dose that produces a therapeutic effect in 50% of the population. A **higher TI** indicates a wider "margin of safety," meaning there is a large gap between the effective dose and the toxic dose (e.g., Penicillin). A **lower TI** indicates a narrow margin of safety, requiring precise dosing and frequent monitoring (e.g., Lithium, Digoxin). **2. Why Other Options are Incorrect:** * **B. Efficacy:** Refers to the maximum response (Emax) a drug can produce, regardless of dose. It is a measure of a drug's effectiveness, not safety. * **C. Potency:** Refers to the amount of drug (dose) required to produce an effect of a given intensity (usually measured by EC₅₀). A more potent drug requires a smaller dose but is not necessarily safer. * **D. Selectivity:** Refers to a drug’s ability to affect a particular receptor or target without affecting others. While related to side effects, it is not the formal definition of the Therapeutic Index. **3. High-Yield Clinical Pearls for NEET-PG:** * **Drugs with Narrow Therapeutic Index (Mnemonic: "Warning! Death Is Likely"):** **W**arfarin, **D**igoxin, **I**nsulin, **L**ithium. Also include Theophylline, Phenytoin, and Aminoglycosides. * **Therapeutic Window:** The range of drug concentrations in which a probability of efficacy is high and the probability of toxicity is low. * **Certain Safety Factor:** A more rigorous index calculated as **LD₁ / ED₉₉**. It ensures that the dose effective for almost everyone is still safe for the most sensitive individual.

Question 35: A partial agonist has:

A. High affinity but low intrinsic activity (Correct Answer)
B. High affinity but no intrinsic activity
C. Low affinity but high intrinsic activity
D. Low affinity and low intrinsic activity

Explanation: ### Explanation To understand this concept, we must distinguish between **Affinity** (the ability of a drug to bind to a receptor) and **Intrinsic Activity/Efficacy** (the ability of a drug to activate the receptor and produce a biological response). **1. Why Option A is Correct:** A **Partial Agonist** is a drug that binds to a receptor (High Affinity) but produces a sub-maximal response even when all receptors are occupied (Low Intrinsic Activity). On a scale of 0 to 1, the intrinsic activity of a partial agonist is between **0 and 1**. Because it competes for the same site as a full agonist but produces a weaker response, it can act as a **functional antagonist** in the presence of a full agonist. **2. Why Other Options are Incorrect:** * **Option B (High affinity, no intrinsic activity):** This describes a **Competitive Antagonist**. It binds to the receptor but has an intrinsic activity of **0**, producing no response on its own. * **Option C & D (Low affinity):** Affinity determines the potency of a drug, not its classification as an agonist or antagonist. A drug can have low affinity and still be a full agonist if it produces a maximal response once bound. **3. NEET-PG High-Yield Clinical Pearls:** * **Intrinsic Activity Values:** Full Agonist = 1; Partial Agonist = 0 to 1; Antagonist = 0; Inverse Agonist = -1. * **Clinical Examples:** * **Pindolol:** A partial agonist at $\beta$-receptors; used in hypertension to prevent excessive bradycardia. * **Buprenorphine:** A partial $\mu$-opioid agonist used in opioid withdrawal; it has a "ceiling effect" on respiratory depression. * **Varenicline:** A partial agonist at nicotinic receptors used for smoking cessation. * **Aripiprazole:** A partial D2 agonist used in schizophrenia.

Question 36: Which of the following conditions is most likely due to a pharmacogenetic cause?

A. Hypoglycemia caused by insulin
B. Tachycardia caused by albuterol
C. Metoclopramide-induced muscle dystonia
D. Primaquine-induced hemolytic anemia (Correct Answer)

Explanation: **Explanation:** **1. Why Option D is Correct:** Primaquine-induced hemolytic anemia is a classic example of a **pharmacogenetic disorder** caused by **Glucose-6-Phosphate Dehydrogenase (G6PD) deficiency**. G6PD is an enzyme essential for maintaining the levels of reduced glutathione in red blood cells (RBCs), which protects them from oxidative stress. Primaquine is an oxidizing drug; in G6PD-deficient individuals, the RBCs cannot neutralize the oxidative stress, leading to hemoglobin denaturation (Heinz bodies) and subsequent hemolysis. This is an X-linked recessive trait, making it a genetically determined abnormal response to a drug. **2. Why Other Options are Incorrect:** * **Option A & B:** Hypoglycemia from insulin and tachycardia from albuterol are **Type A (Augmented)** adverse drug reactions. These are predictable, dose-dependent extensions of the drug’s primary pharmacological action and occur in most individuals if the dose is high enough. * **Option C:** Metoclopramide-induced muscle dystonia is an **Extrapyramidal Side Effect (EPS)** due to central dopamine (D2) receptor blockade. While some individuals are more sensitive, it is generally considered a predictable side effect of the drug's mechanism rather than a specific genetic polymorphism. **3. Clinical Pearls for NEET-PG:** * **Other drugs causing hemolysis in G6PD deficiency:** Sulfonamides, Dapsone, Nitrofurantoin, and Chloroquine. * **Succinylcholine Apnea:** Caused by a genetic deficiency of **Pseudocholinesterase** (Butyrylcholinesterase). * **Slow Acetylators (NAT2 polymorphism):** Increased risk of peripheral neuropathy with **Isoniazid** and SLE-like syndrome with **Hydralazine/Procainamide**. * **Malignant Hyperthermia:** Genetic mutation in **Ryanodine receptors (RyR1)** triggered by Halothane or Succinylcholine.

Question 37: What does the potency of a drug refer to?

A. Affinity of drug to bind to a receptor
B. Affinity of drug that binds to receptors and activates it
C. The dose required to produce a specific response (Correct Answer)
D. The maximum response a drug can produce

Explanation: **Potency** refers to the amount or **dose** of a drug required to produce an effect of a given intensity. On a Dose-Response Curve (DRC), potency is represented by the position of the curve along the x-axis (dose) [4]. A drug that produces the same effect at a lower dose is considered "more potent" [2]. **Analysis of Options:** * **Option C (Correct):** Potency is mathematically expressed as the **$EC_{50}$** (the concentration required to produce 50% of the maximum response) [1, 3]. The lower the $EC_{50}$, the higher the potency. * **Option A (Incorrect):** This describes **Affinity**, which is the tendency of a drug to bind to its receptor. While affinity influences potency, they are not synonymous. * **Option B (Incorrect):** This describes **Intrinsic Activity** (Efficacy). It is the ability of a drug to trigger a pharmacological response after binding to the receptor. * **Option D (Incorrect):** This describes **Efficacy** ($E_{max}$). Efficacy is the maximal clinical response a drug can achieve. In clinical practice, efficacy is generally more important than potency. **NEET-PG High-Yield Pearls:** 1. **Potency vs. Efficacy:** On a graph, a shift to the **left** indicates increased potency. A shift **upwards** indicates increased efficacy. 2. **Clinical Relevance:** Potency is rarely a decisive factor in drug choice; it only determines the milligram weight of the tablet [2]. For example, 5mg of Amlodipine is more potent than 50mg of Atenolol, but they are used for different clinical indications [2]. 3. **Relative Potency:** If Drug A produces a response at 10mg and Drug B produces the same response at 100mg, Drug A is 10 times more potent than Drug B [2].

Question 38: Which of the following statements is false?

A. Lipid-soluble drugs tend to have a higher volume of distribution.
B. The volume of distribution is reduced by high plasma protein binding.
C. Loading dose is equal to the volume of distribution multiplied by the target plasma concentration. (Correct Answer)
D. Half-life is constant in first-order kinetics.

Explanation: ### Explanation This question tests the fundamental principles of pharmacokinetics. The statement in **Option C** is technically incomplete and considered "false" in a clinical context because it ignores **bioavailability (F)**. #### 1. Why Option C is the Correct Answer (False Statement) The formula for **Loading Dose (LD)** is: $$LD = \frac{V_d \times C_p}{F}$$ Where $V_d$ is the Volume of Distribution, $C_p$ is the Target Plasma Concentration, and $F$ is Bioavailability. For intravenous drugs, $F = 1$, making the statement true. However, for any other route (like oral), the dose must be adjusted for bioavailability. Without specifying the route, the general formula must include $F$. #### 2. Analysis of Other Options * **Option A (True):** Lipid-soluble drugs easily cross biological membranes and distribute into adipose tissue and intracellular compartments, leading to a **high $V_d$** (e.g., Chloroquine). * **Option B (True):** Drugs that are highly bound to plasma proteins (like albumin) remain confined to the vascular compartment. This results in a **low $V_d$**. * **Option D (True):** In **First-order kinetics**, a constant *fraction* of the drug is eliminated per unit time. Therefore, the **half-life ($t_{1/2}$)** remains constant regardless of the plasma concentration. #### 3. High-Yield NEET-PG Pearls * **Maintenance Dose (MD):** Calculated using **Clearance (CL)**: $MD = \frac{CL \times C_{ss} \times \tau}{F}$ (where $\tau$ is the dosing interval). * **Zero-order Kinetics:** Half-life is **not** constant; it depends on the concentration (e.g., Ethanol, Phenytoin, Aspirin at high doses). * **Volume of Distribution:** It is a theoretical volume, not a physical one. If $V_d$ exceeds total body water (~42L), it indicates the drug is sequestered in tissues.

Question 39: Which of the following drugs acts on -2 receptors?

A. Phenylephrine
B. Midodrine
C. Clonidine (Correct Answer)
D. Methoxamine

Explanation: **Explanation:** The question tests your knowledge of sympathomimetic and sympatholytic drugs and their receptor selectivity. **Correct Answer: C. Clonidine** Clonidine is a prototypical **selective $\alpha_2$-adrenergic agonist**. It acts primarily on presynaptic $\alpha_2$ receptors in the vasomotor center of the medulla. Activation of these receptors inhibits the release of norepinephrine, leading to a decrease in sympathetic outflow to the heart and peripheral vasculature. This results in a reduction in blood pressure, making it a centrally acting antihypertensive. **Analysis of Incorrect Options:** * **A. Phenylephrine:** This is a selective **$\alpha_1$-agonist**. It is commonly used as a nasal decongestant and a vasopressor to increase blood pressure without affecting the heart rate directly (though it may cause reflex bradycardia). * **B. Midodrine:** This is an orally active **$\alpha_1$-agonist** (a prodrug converted to desglymidodrine). It is primarily used in the treatment of symptomatic orthostatic hypotension. * **D. Methoxamine:** This is another selective **$\alpha_1$-agonist** used primarily as a vasopressor to treat hypotension during anesthesia. **High-Yield Clinical Pearls for NEET-PG:** * **$\alpha_2$ Agonists:** Apart from Clonidine, other important $\alpha_2$ agonists include **Methyldopa** (drug of choice for hypertension in pregnancy), **Dexmedetomidine** (used for sedation in ICUs), and **Tizanidine** (a central muscle relaxant). * **Clonidine Withdrawal:** Abrupt cessation of clonidine can lead to a "rebound hypertensive crisis" due to a sudden surge in catecholamines. * **Diagnostic Use:** The **Clonidine Suppression Test** is used in the diagnosis of Pheochromocytoma (clonidine fails to suppress catecholamine levels in affected patients).

Question 40: Atrial Natriuretic Peptide receptors belong to which of the following types of receptors?

A. G-protein coupled receptors
B. Ligand-gated guanylyl cyclase receptor (Correct Answer)
C. Tyrosine kinase receptors
D. JAK-STAT receptors

Explanation: **Explanation:** **Atrial Natriuretic Peptide (ANP)** and Brain Natriuretic Peptide (BNP) exert their physiological effects by binding to **Natriuretic Peptide Receptor-A (NPR-A)**. This receptor is a classic example of a **transmembrane (membrane-bound) Guanylyl Cyclase**. When ANP binds to the extracellular domain, it directly activates the intracellular catalytic domain of the receptor, which converts GTP into **cyclic GMP (cGMP)**. cGMP then activates Protein Kinase G (PKG), leading to vasodilation and natriuresis. This is distinct from soluble guanylyl cyclase, which is activated by Nitric Oxide. **Analysis of Incorrect Options:** * **A. G-protein coupled receptors (GPCRs):** These utilize secondary messengers like cAMP or $IP_3/DAG$. While many hormones use GPCRs, ANP does not. * **C. Tyrosine kinase receptors:** These (e.g., Insulin, EGF) involve autophosphorylation of tyrosine residues. ANP receptors lack tyrosine kinase activity. * **D. JAK-STAT receptors:** These are used by cytokines and Growth Hormone. They do not have intrinsic enzymatic activity but recruit cytosolic Janus Kinases (JAKs). **High-Yield Clinical Pearls for NEET-PG:** 1. **Dual Role:** ANP receptors are both the receptor and the enzyme (Guanylyl Cyclase). 2. **Sacubitril:** A Neprilysin inhibitor used in Heart Failure (ARNI) that prevents the degradation of ANP/BNP, thereby increasing cGMP levels. 3. **Nesiritide:** A recombinant form of BNP used in acute decompensated heart failure, acting via this same cGMP pathway. 4. **Other cGMP-linked drugs:** Sildenafil (PDE-5 inhibitor) prevents the breakdown of the cGMP generated by these pathways.

Practice by Chapter

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